1. Field of the Invention
The present invention relates to a mixer circuit used in frequency conversion, quadrature modulation, quadrature demodulation and the like and a radio communication device using the mixer circuit.
2. Description of the Related Art
There is known a balanced mixer as one of mixer circuits used in a frequency converter or the like. U.S. Pat. No. 6,748,204 discloses a balanced mixer configured so that an impedance element, which causes a low-frequency signal current to pass through but hardly causes a signal current at a frequency near a high-frequency input signal frequency to pass through, is connected to common source terminals of a switching stage.
Further, Behzad Razavi, “A 60-GHz CMOS Receiver Front-End,” IEEE Journal of Solid-State Circuits, Volume 41, Issue 1, pp. 17-22 discloses a balanced mixer configured so that an input transistor is connected to common source terminals of a switching stage, one end of an inductor corresponding to the impedance element is connected to the common source terminals, and the other end of the inductor is connected to a capacitor and a current source. A value of the inductor is selected so that a parasitic capacitance between the common source terminals and a ground and the inductor make a parallel resonant circuit.
In this way, if the inductor and the parasitic capacitance make the parallel resonance, a high-frequency current signal generated in the input transistor hardly diverges to the parasitic capacitance but is input to the switching stage. Therefore, the mixer circuit can acquire high gain. Moreover, since the current source connected to the other end of the inductor limits the current carried across the switching stage, thermal noise and flicker noise generated in transistors of the switching stage are reduced. Besides, the balanced mixer can operate at a low power supply voltage by reducing voltage drop resultant from load resistance.
If a double balanced mixer operable for differential input signals is to be realized using the circuit disclosed in U.S. Pat. No. 6,748,204, it suffices to double an input-stage circuit and the switching stage. However, the prior art circuit includes two problems, one being even order distortion and DC offset which occur from the mixer circuit itself, and the other being DC offset, noise and degradation of distortion characteristic, which occur when the common mode signal leaked from outside is input to the circuit.